Air filters, as a means of removing impurities in surrounding air are well known in the prior art. Air filters are widely used on internal combustion engines, for example, to remove impurities from the air before the air enters an engine for combustion. Air filters are also used in vacuum units for cleaning and for air purification of work environments. Such air filters typically have air drawn through a filter medium, wherein particulate impurities in the air, such as smoke, dust and dirt are removed. After a period of time, the filters become less efficient as they become clogged with the trapped impurities. It is then necessary to remove and replace the filter or provide some means for cleaning the filter.
A number of methods are known and used to clean air filters. Such methods usually use air which is blown through the filters. The air flow is generally in the form of air pressure, air suction, air vacuum, or some combination. The most common filter is the annular rotary drum type, generally forming a cylindrical shape. It has been found that cleaning of such a filter by vacuum or suction is an inefficient method for removing particulate matter from the exterior surface of the filter. Generally, the prior art shows that a cleaning arrangement of the type in which vacuum or suction is used from the exterior surface requires more vacuum pressure than other types of cleaning arrangements. Further, a device used to clean an annular shaped filter with suction is necessarily a complicated device. Unless a standard manual vacuum is used, the device must be able to circumferentially traverse the exterior surface of the filter, usually by rotating about the axis of the filter. The vacuum cleaning device may be simplified if the suction arrangement is stationary and the filter rotates. However, in such arrangement it becomes necessary to provide a rather complicated filter container in order that both the filter and its supports can be rotated across the vacuum. Finally, vacuum pressure is usually more difficult to control properly. The desired suction is not always necessarily attainable or constant through a fixed nozzle, especially as particulate matter is drawn from the filter surface into the suction line reducing the vacuum pressure being applied to the filter surface.
Vibration is another commonly known and used method of cleaning an air filter. Vibration usually involves shaking the filter in order to loosen trapped impurities. This may be done to the filter while in place or after removing the filter from the cleaning unit. In certain arrangements, vibrations are induced by a striking mechanism in which the filter is struck repeatedly. Such an arrangement is undesirable in that wear is created on both the striking mechanism and the filter. Further, when the filter is shaken or struck in the cleaning unit, unnecessary vibration is induced in the cleaning unit itself.
Finally, another method of cleaning an air filter is using forced air flow directed outwardly from the interior annular surface of a cylinder shaped filter to automatically dislodge accumulated dust and other particulates by reverse air cleaning. Thus, particles which were generally trapped on the air filter along the outer perimeter during operation of the vacuum cleaning device are removed as air is forced through the filter to dislodge the particulate from the reverse direction. Such devices generally require either rotation of the filter or rotation of the cleaning unit, and air is normally directed to only localized areas of the filter at any one time. Further, the devices shown in the prior art require a mechanical drive arrangement to rotate either the filter or cleaning device. Such a device is disclosed in Shackleton et al., U.S. Pat. No. 3,936,902 herein incorporated by reference. Each of the above cleaning solutions is impractical and inefficient to clean the filter cartridges inside small, portable dust and smoke extraction units. Such units have too little space to accommodate complex cleaning assemblies and the associated drive motors. Additionally, drive motors add undesirable weight to portable vacuum units. A complex cleaning unit and/or drive motors further complicate the maintenance and reliability of vacuum units having an integral cleaning unit. Where no cleaning unit is provided, it is necessary to regularly change filters as they become clogged, and vacuum unit efficiency is therefore reduced. This increases down time and reduces efficiency in the work area. Further, new filter costs significantly increase the cost of operating such small vacuum units.